This study was designed to take advantage of the unprecedented post- vaccinal epidemic of "serum hepatitis" in the U S Army in 1942, the record systems of the Army and the Department of Veterans Affairs, and the access to the IRS list of taxpayer addresses that legislation has provided for occupational research. Three cohorts were formed: men with clinical disease in 1942; men who received yellow fever vaccine from numbered lots known to have been contaminated; and controls with no exposure to the contaminated vaccine. The first phase of the study was a serologic survey of about 200 men from each of the three groups. Later phases were directed at the relation between subclinical infection with the hepatitis B virus (HBV) and liver disease, especially cirrhosis, and liver cancer. The entire study has now been completed, with the final paper having been submitted for publication.